The present invention is directed to improved fibrous absorbent structures having separate layers for fluid acquisition, distribution , and storage. The acquisition layer contains latex-bonded synthetic fibers, and is useful in providing improved disposable absorbent products, such as diapers, adult incontinence pads, and sanitary napkins.
Absorbent articles such as disposable diapers, adult incontinence pads, sanitary napkins, and the like, are generally provided with an absorbent core, or storage layer, to receive and retain bodily liquids. The absorbent core is usually sandwiched between a liquid pervious top sheet, whose function is to allow the passage of fluid to the core, and a liquid impervious backsheet which contains the fluid and prevents it from passing through the absorbent article. An absorbent core (e.g., for diapers and adult incontinence pads) typically includes fibrous batts or webs constructed of defiberized, loose, fluffed, hydrophilic, cellulosic fibers. The core may also include superabsorbent polymer (SAP) particles, granules, flakes or fibers. In addition, an absorbent article may contain a distribution layer that aids in transporting liquid quickly from the acquisition layer to the storage layer of the core.
In recent years, market demand for thinner and more comfortable absorbent articles has increased. Such articles may be obtained by decreasing the thickness of the diaper core, by reducing the amount of fibrous material used in the core while increasing the amount of SAP particles, and by calendering or compressing the core to reduce caliper and hence, increase density. However, higher density cores do not absorb liquid as rapidly as lower density cores because densification of the core results in smaller effective pore size. Accordingly, to maintain a suitable liquid absorption rate, it is necessary to provide a lower density layer having a larger pore size above the high density absorbent core to increase the rate of uptake of liquid discharged onto the absorbent article. The low density layer is typically referred to as an acquisition layer.
The storage layer portion of a disposable diaper core for example, is generally formed in place, during the converting process, from loose, fluffed cellulose. Superabsorbent powder is blended with the fluff cellulose fibers as the absorbent core is formed on the diaper converting line. Such cellulose material is generally not available in preformed roll form because it exhibits insufficient web strength, due to the lack of interfiber bonding or entanglement.
The acquisition layer portion of a disposable diaper is generally a carded synthetic staple fiber web that is thermally bonded, latex bonded, or point bonded. Typical staple fibers for acquisition layers are crimped polyester (PET) or polypropylene fibers that have a size of 6 to 15 denier and a length of at least 40 mm. The acquisition layer is formed, bonded and slit as a homogenous rolled good on a dedicated nonwoven textile production line. The slit roll of acquisition layer material is subsequently unrolled onto the diaper converting line where it is affixed on top of the absorbent core and below the topsheet. Examples of commercial infant diapers with a bonded carded staple fiber are Huggies Diapers produced by Kimberly-Clark Corp. (Dallas, Tex.) and private label diapers produced by Paragon Trade Brands (Atlanta, Ga.).
Modern infant disposable diaper converting machines have become extremely complex as more and more features such as elastication and multiple nonwovens have been implemented to improve diaper performance. This complexity has created significant raw material handling issues and a resultant loss of converting line productivity. There is a need to replace the bulky and cumbersome fluff pulp and superabsorbent powder forming systems with a single material that can simply be fed directly into the converting line from a roll or other suitable compact package. Because the acquisition layer and absorbent core are placed together in the final product, it can maximize the efficiency of the converting operation to combine the fluid acquisition layer and the absorbent core in a single material.
Ultra-thin feminine napkins are generally produced from roll-goods based nonwoven material. Such a roll of preformed absorbent core material is unwound directly onto the absorbent article converting equipment without the defiberization step required for fluff-based products, such as diapers and incontinence pads. The nonwoven web is typically bonded or consolidated in a fashion that gives it sufficient strength to be handled in the converting process. The web may also contain SAP particles.
The web consolidation mechanisms used in the roll-goods approach to making preformed cores provide strength and dimensional stability to the web. Such mechanisms include latex bonding, bonding with thermoplastic or bicomponent fibers or thermoplastic powders, hydroentanglement, needlepunching, carding, or the like.
One embodiment of a structure having an acquisition layer and a distribution layer (an xe2x80x9cADLxe2x80x9d) typically found on die-cut feminine hygiene pads is an airlaid cellulose web bonded with an aqueous binder resin that has been dried and cured. Airlaid materials typically retain up to 16 g of fluid per gram of material against gravity under negligible load. Thus, an ADL can acquire a surge of fluid within the absorbent product until the superabsorbent particles in the absorbent core can absorb the retained fluid out of the airlaid cellulose ADL and into final storage containing superabsorbent particles.
An example of a conventional airlaid cellulose material is Vicell 6002 (Buckeye Technologies Inc., Memphis Tenn.), which is a 105 gsm (grams per square meter) airlaid cellulose non-woven bonded with a vinyl acetate binder resin. Vicell 6002 is prepared by spraying an aqueous emulsion of the vinyl acetate binder resin onto the airlaid cellulose web followed by drying and curing in a hot air oven. It is used commercially in an ADL for feminine hygiene pads.
The disadvantage of certain commercially available airlaid cellulose structures is that they may collapse under normal use. This typically occurs when the structure is compressed by the weight of the wearer and particularly when the structure becomes wet. This structural collapse significantly reduces the fluid acquisition rate of the absorbent product and thus increases the chance of leakage. When a completely or partially fluid saturated airlaid cellulose structure collapses, the fluid escapes from the ADL and the product feels wet against the wearer""s skin.
There is a need for thin absorbent core material which facilitates fluid transport from an acquisition zone to a storage zone, has a high absorbent capacity in use, and can be delivered in roll-goods form to simplify the manufacturing and converting processes.
Applicants have now surprisingly discovered an improved ADL containing at least two discrete layers, the top and the bottom layer, which overcomes the above-described disadvantage of commercially available products. The top layer (i.e., the layer in contact with the skin of wearer) of the ADL of the present invention is highly porous, thus preventing the collapse of the structure and minimizing the leakage problem.
The present invention provides a highly absorbent, high-bulk low density article having an absorbent structure comprising a liquid acquisition and, optionally, a distribution layer and a fibrous liquid storage layer in communication with the acquisition layer. The storage layer contains SAP particles, latex-bonded fibers, thermally bonded-fibers, or a combination thereof.
In one embodiment, the invention relates to an improved acquisition and distribution layer (ADL) having at least two layers for use in disposable absorbent products, a top (acquisition) layer in contact with the wearer of the absorbent product, and a bottom (distribution) layer between the top layer and a storage layer. Thus, according to one aspect of the invention, an improved ADL with a highly porous acquisition layer is provided.
In another aspect of the invention, an airlaid rolled good containing the ADL of the invention, and a method for its production is provided.
In yet another aspect of the invention, a disposable absorbent product containing the ADL of the invention, and a method for its production is provided.
The absorbent structure of the invention has the following advantages: (i) it is highly absorbent as it is made of a low-density material and has high-bulk properties; (ii) the layers are uniform due to the manner in which superabsorbent particles are deposited into the layers providing an absorbent web, and thus provide increased absorbent potential of the article; (iii) the absorbent article permits more economical means for providing an absorbent article because the function of multiple materials are combined into a single roll; and (iv) the wettability of the article can be adjusted as a result of (or by) surfactant addition during the acquisition layer forming process.
All patents, patent applications, and literature references cited herein are hereby incorporated by reference in their entirety.
The present invention provides an improved fibrous absorbent structure which contains a liquid acquisition layer of lower density and a fibrous storage layer of relatively higher density. The structure is a composite including at least these two layers which confer upon the structure the ability to acquire and distribute fluids through the density gradient. The acquisition layer is capable of rapidly acquiring liquid from insult. The storage layer absorbs and stores the liquid acquired from the acquisition layer.
In addition, the invention preferably contains a distribution layer, which in combination with the fluid acquisition layer provides an improved ADL, containing at least two layers, a top acquisition layer and a distribution layer. When used in a disposable absorbent product, the acquisition layer is closer to the skin of the wearer and away from the storage layer; the distribution layer is closer to the storage layer and away from the skin of the wearer. The acquisition layer provides rapid fluid acquisition under load. The distribution layer provides z-direction capillary force to pull fluid into the absorbent storage layer, away from the skin of the wearer, to provide temporary fluid immobilization, and to act as a conduit for fluid drawn into the unsaturated portion of the storage layer. The absorbent structure of the invention has high absorbent capacity and is particularly useful as an absorbent core for disposable absorbent articles such as diapers, adult incontinence pads and briefs, and feminine sanitary napkins, and the like.
The fiberized fluff cellulose fibers used in the ADL or storage layer of the composite structure of the present invention may be selected from wood cellulose such as Foley fluff, cotton linter pulp, chemically modified cellulose such as crosslinked cellulose fibers or highly purified cellulose fibers, such as Buckeye HPF.
The present invention makes use of the unexpected discovery that a latex-bonded, synthetic fiber in the acquisition layer provides an absorbent structure having improved acquisition and retention characteristics (i.e., absorbency) compared with an absorbent structure employing an acquisition layer lacking such fibers. The advantage of using synthetic fibers is that such fibers maximize the surface dryness of the absorbent product. Any synthetic fiber, including polyester fibers, such as polyethylene terepthalate (PET), polypropylene, nylon and acrylic, and combinations thereof, may be used provided that the fiber has the property of forming large pores resistant to collapse when the layer is wet.
The melting point of the synthetic fiber should be taken into consideration during the manufacturing process and the temperature should be adjusted to avoid melting of the fiber. For purposes of the present disclosure, xe2x80x9clarge porexe2x80x9d means a pore larger, and more resistant to collapse, than the pore formed by cellulose fibers. Because large pores contained within a synthetic fiber matrix resist collapse under pressure when wet, the top layer can rapidly acquire a surge of fluid as it passes through the liner or top sheet of the absorbent product. The size of the pore will depend on the composition of the fiber, size of the fiber (i.e., fiber diameter), resiliency of the fiber, and resiliency of the latex. A person of skill in the art may optimize the pore size to suit any particular need using general knowledge in the art (see, for example, U.S. Pat. Nos. 5,569,226 and 5,505,719 issued to Cohen) and routine experimentation.
The synthetic fibers of the top layer are bonded with an aqueous dispersion (emulsion) of a natural or a synthetic polymer latex. Any latex may be used in the invention. The synthetic polymer may be, for example, a polymer or copolymer of alkylacrylates, vinyl acetate, or styrene-butadiene. Other polymers known in the art may be used. For purposes of industrial hygiene and elimination of a solvent recycling step, the synthetic latexes can be applied as an aqueous based emulsion rather than an organic solvent emulsion. In the present invention, the preferred matrix fibers of the acquisition layer are 3 to 20 denier crimped PET fibers with a cut length of 3 to 15 mm.
The distribution layer of the improved ADL preferably contains latex and/or thermal bonded cellulose fibers. Any fluff cellulose fibers may be used in this layer, preferably wood fibers such as airlaid-fluff cellulose, chemically modified cellulose fibers, (e.g., cross-linked cellulose fibers), highly purified cellulose, cotton linter fibers, or blends thereof. For bonding purposes, the latex dispersions used for bonding the fibers of the top layer may be used. Alternatively, or in combination with a latex binder, thermoplastic fibers or powder may be used for bonding upon heating to the melting point of the thermoplastic fiber or powder. Bicomponent fibers having a PET core surrounded by a polyethylene sheath, e.g., Hoechst-Trevira Type-255 (Charlotte, N.C.), and polyethylene powder may be used.
The distribution layer of the improved ADL provides both a temporary retention zone and a liquid distribution channel into the final storage layer. The cellulose fibers of this layer form a microporous medium that spontaneously distributes fluid from the point of fluid insult to unsaturated portions of the distribution layer via a combination of surface tension driving force and gravity. Once the spreading fluid from insult contacts an unsaturated portion of the storage layer, which has a higher surface tension than the ADL, the fluid within the distribution layer flows into the storage layer until a surface tension equilibrium is reached. The higher surface tension of the storage layer can be generated by providing higher density cellulose fiber and/or superabsorbent particles. Thus the distribution layer becomes both a fluid reservoir for, and a flow channel to, the storage layer. The flow channel function is particularly important in certain thin absorbent pads where the ADL covers a significantly greater surface area than the absorbent core or storage layer.
Optionally, other ingredients such as surfactants, pigments, and opacifiers may be added to the acquisition or distribution layers without affecting absorbency.
The basis weight of the ADL layer of the invention may range from about 30 to 150 gsm, preferably from about 60 to 100 gsm, and most preferably about 80 gsm. In one embodiment, the basis weight of each of the acquisition and the distribution layer of the ADL may range from about 15 to 60 gsm. Preferably, the basis weight of each layer is at minimum about 25% of the total ADL basis weight. In one embodiment, the top layer is from about 25 to 50% of the total ADL basis weight.
The ADL of the present invention may contain an optional middle layer. The middle layer may contain 100% fluff cellulose and/or chemically modified cellulose fibers or have a fiber composition that is a blend of synthetic fibers and cellulose fibers.
In one embodiment of the ADL, the acquisition layer contains about 80-90% by weight of 6.7 dtex (wt/length of fiber) in size by 6 mm in cut length polyester (PET) fiber bonded with 10-20% by weight of an aqueous binder resin. The bottom layer contains 80-90% of fiberized fluff cellulose fibers that are bonded using 10-20% of an aqueous binder. The fiberized fluff cellulose fibers may contain wood cellulose such as Buckeye Foley fluff (Buckeye Technologies Inc.), cotton linter pulp such as Buckeye HPF (Buckeye Technologies Inc.), or chemically modified cellulose such as cross-linked cellulose fibers. In another embodiment, the top and bottom layers of the ADL are as above, but a middle layer containing a blend of PET and cellulosic fibers is also present. In this embodiment the top layer is at least 10% of the total ADL weight and the bottom layer is no more than 50% of the total ADL basis weight.
The preferred overall basis weight range of the composite absorbent structure of the invention is 100-500 grams per square meter (gsm). The composite absorbent structures include a fluid acquisition layer and a fluid distribution layer (described above) and a fluid storage layer. The fluid storage layer is below the fluid distribution layer and is comprised of a fluff cellulose or chemically modified fluff cellulose matrix fibers, a superabsorbent polymer (SAP), and a bonding element. The bonding element is preferably bicomponent fibers in the concentration of 5 to 20% but may also include thermoplastic powders. Preferably, the SAP content is 10-70% by weight of the absorbent structure.
As used herein, xe2x80x9csuperabsorbent polymerxe2x80x9d or xe2x80x9cSAPxe2x80x9d means any suitable hydrophilic polymer that can be mixed with fibers of the present invention. A superabsorbent polymer is a water soluble compound that has been cross-linked to render it water insoluble but still swellable to at least about 15 times its own weight in physiological saline solution. These superabsorbent materials generally fall into 3 classes, namely starch graft copolymers, cross-linked carboxymethylcellulose derivatives, and modified hydrophilic polyacrylates. Examples of absorbent polymers include hydrolyzed starch-acrylontrile graft co-polymer, saponified acrylic acid ester-vinyl co-polymer, modified cross-linked polyvinyl alcohol, neutralized cross-linked polyacrylic acid, cross-linked polyacrylate salt, and carboxylated cellulose. The preferred superabsorbent materials, upon absorbing fluids, form hydrogels.
The superabsorbent polymer materials have relatively high gel volume and relatively high gel strength as measured by the shear modulus of the hydrogel. Such preferred materials also contain relatively low levels of polymeric materials which can be extracted by contact with synthetic urine. Superabsorbent polymers are well-known and are commercially available. One example is a starch graft polyacrylate hydrogel marketed under the name IM1000 (Hoechst-Celanese, Portsmouth, Va.). Other commercially available superabsorbent polymers are marketed under the trademark Sanwet (Sanyo Kasei Kogyo Kabushiki, Japan), Sumika Gel (Sumitomo Kagaku Kabushiki Haishi, Japan), Favor (Stockhausen, Garyville, La.) and the ASAP series (Chemdal, Aberdeen, Miss.). Superabsorbent particulate polymers are also described in detail in U.S. Pat. No. 4,102,340 and U.S. Pat. No. Re. 32,649. An example of a suitable SAP is surface cross-linked acrylic acid based powder such as Stockhausen 9350 or SX FAM 70 (Greensboro, N.C.).
The preferred basis weight range(s) and SAP content may vary with the intended application. For feminine hygiene and light capacity adult incontinence applications, for example, the basis weight and SAP content will tend to be toward the lower end of the ranges indicated in Table 1. For infant diaper and heavy capacity adult incontinence applications, the preferred basis weight and SAP content will tend to be toward the high end of the specified range in Table 1.
Multiple matrix fibers can be used in an absorbent article of the invention, however, it is preferred that collectively the matrix fibers constitute most of the fibers in the material (e.g., at least 75%). The term matrix fiber as used herein, refers to a synthetic or cellulosic fiber that does not melt or dissolve to any degree during the forming or bonding of an air-laid absorbent structure. The terms xe2x80x9cthermal bondingxe2x80x9d or xe2x80x9cthermalxe2x80x9d herein refer to the blending of thermoplastic material (e.g., bonding methods listed in Table 1) in which the matrix fiber(s) and SAP bond the absorbent layers when heat is applied.
Examples of suitable thermoplastic materials include thermoplastic microfibers, thermoplastic powders, bonding fibers in staple form, and bicomponent staple fibers. Bicomponent staple fibers are characterized by a high melt temperature core polymer (typically polyethylene terephthalate (PET) or polypropylene) surrounded by a low melt temperature sheath polymer (typically polyethylene, modified polyethylene, or copolyesters). In the preferred embodiments of this invention, bicomponent fibers provide the means of thermal bonding.
Table 1 provides a general outline of an embodiment of the invention.
In one embodiment for feminine hygiene and light adult incontinence products, the absorbent product of the invention contains an acquisition layer, a distribution layer and a storage layer, having a total basis weight of 120 gsm to 290 gsm. The acquisition layer, comprising latex bonded PET matrix fibers, has a 20 to 60 gsm total basis weight. The matrix fibers are 6 to 15 denier in size, 3 to 12 mm in length and have 2 to 5 crimps/cm. The latex is an emulsion of ethylene vinyl acetate, styrene-butadiene, or acrylic polymer. The latex binder is about 5 to 25% of the weight of the acquisition layer. The distribution layer, comprising thermal bonded fluff cellulose or chemically modified fluff cellulose fibers, has a 30 to 90 gsm total basis weight. The cellulose fiber is thermally bonded with 5 to 20% by weight of sheath/core bicomponent fiber (e.g., 3 denier T-255 bicomponent fiber from Hoechst-Celanese, Charlotte, N.C.). The storage layer of thermal bonded fluff cellulose with 20 to 50% SAP has a 70 to 130 gsm total basis weight. The fluff cellulose/SAP mixture is thermally bonded with 5 to 10% by weight of sheath/core bicomponent fiber. Specific embodiments are described in Examples 3, 9, 14, 15, 16.
In another embodiment, which can be employed in infant diapers and heavy adult incontinence products, the acquisition layer, distribution layer, and storage layer have a total basis weight of 300 gsm to 500 gsm. The acquisition layer, comprising latex bonded PET matrix fibers, has a 20 to 60 gsm total basis weight. The matrix fibers are 6 to 15 denier in size, 3 to 12 mm in length and have 2 to 5 crimps/cm. The latex is an emulsion of ethylene vinyl acetate, styrene-butadiene, or acrylic polymer, and is about 5 to 25% of the weight of the acquisition layer. The distribution layer, comprising thermally bonded fluff cellulose or chemically modified fluff cellulose fibers, has a 30 to 100 gsm total basis weight. The cellulose fiber is thermally bonded with 5 to 20% by weight of sheath/core bicomponent fiber. The storage layer of thermally bonded fluff cellulose with 40 to 75% SAP has a 250 to 340 gsm total basis weight. The fluff cellulose/SAP mixture is thermally bonded with 5 to 10% by weight of sheath/core bicomponent fiber. Specific embodiments are described in Examples 8 and 10.
An absorbent article of the invention can be made in one continuous process utilizing air forming equipment such as equipment sold by MandJ Fibertech (Horsens, Denmark) or Dan-Web (Aarhus, Denmark). The bottom or cellulose fiber layer is formed onto a moving collection wire, and the synthetic fiber layer is airlaid directly on top of the cellulose fiber layer. The resulting composite structure is then passed under an adhesive application station (typically a set of spray nozzles or foam coater) that applies adhesive directly onto the synthetic fiber layer. The material then travels through a hot air oven, or other suitable heating device, to bond the structure. In the preferred embodiments, adhesive is next applied to the cellulose fiber side of the composite structure and the material is passed through a second oven to dry the adhesive. A third heating station may be employed to insure that the adhesive is fully cured. The absorbent structure of the invention may be finally packaged and shipped in roll-goods form.
Preferably, the absorbent structures of the invention are prepared as an airlaid web. The composite material may be manufactured in a continuous operation provided the production line has at least three separate forming heads, a synthetic fiber dosing system capable of handling at least two different synthetic fibers simultaneously, a superabsorbent powder dosing system and a latex adhesive application system.
The airlaid web is typically prepared by disintegrating or fiberizing a cellulose pulp sheet or sheets, typically by hammermill, to provide individualized fibers. The individualized fibers are then air conveyed to forming heads on the airlaid web forming machine. The forming heads include rotating or agitated drums, generally in a race track configuration which serve to maintain fiber separation until the fibers are pulled by vacuum onto a foraminous condensing drum or foraminous forming conveyor (or forming wire). In the MandJ machine, the forming head includes a rotary agitator above a screen. Other fibers, such as a synthetic thermoplastic fiber, may also be introduced to the forming head through a fiber dosing system which includes a fiber opener, a dosing unit and an air conveyor. Where two defined layers are desired, such as a fluff pulp distribution layer and a synthetic fiber acquisition layer, two separate forming heads are provided, one for each type of fiber.
The airlaid web is transferred from the forming wire to a calender or other densification stage to densify the web, increase its strength and control web thickness. The fibers of the web are then bonded by application of a latex spray or foam addition system, followed by drying or curing. Alternatively, or additionally, any thermoplastic fiber present in the web may be softened or partially melted by application of heat to bond the fibers of the web. The bonded web may then be calendered a second time to increase strength or emboss the web with a design or pattern. If thermoplastic fibers are present, hot calendering may be employed to impart patterned bonding to the web. Water may be added to the web if necessary to maintain specified or desired moisture content, to minimize dusting, or to reduce the buildup of static electricity. The finished web is then rolled for future use.
In one embodiment (e.g., Example 1) the acquisition and distribution layer are air formed independent of the fluid storage layer. The composite acquisition/distribution layer is combined with the storage layer at the converting line. This embodiment is useful for absorbent product designs where the storage layer covers a different area than the acquisition/distribution layer. Other embodiments of this type are described in Examples 11, 12, and 13.
The following non-limiting Examples further describe the invention, the scope of which is to be limited only by the claims.